Airborne particles are ever present in the environment, and are of concern for human health, for atmospheric visibility and for climate change. Measurement of the chemical characteristics of these particles, and of the health effects associated with their inhalation is often aided by air-to-air “particle concentrators” that enrich the particle concentration in a portion of the air flow stream.
Virtual impactors are a well known type of “air-to-air” particle concentrator that use a low-velocity sampling probe to sample a particle flow exiting from a nozzle. Because of the mismatch in air velocities, those particles larger than a few micrometers are preferentially captured within the low-velocity sampling probe. This is a commonly used approach for enrichment of particles with diameters above a few micrometers, but is ineffective for the submicrometer and ultrafine particle size range of most interest for atmospheric and health-related studies.
Current “air-to-air” concentrators for small particles couple these traditional virtual impactors with condensational growth. These small particle concentrators use steam injection or warm saturators with cooling and turbulent mixing to condense water on particles present in the flow, enlarging them to supermicrometer-sized droplets prior to entering the virtual impactor. These systems are ineffective for particles below about 30 nm in diameter. Moreover, with long condensation times, these approaches have been shown to have the undesirable effect of changing the particle chemical composition.